1. Field of the Invention
This invention relates to novel oxidation methods and is more particularly concerned with the ionic oxidation of certain phthalide moieties to aromatic polycarboxylic acids, a method for the autoxidation of certain aromatic compounds having ortho dialkyl substituents to said phthalides, particular novel diphthalides produced therefrom, and an improved method for converting said aromatic compounds to said polycarboxylic acids by the combination of the above two oxidation methods.
2. Description of the Prior Art
Autoxidation processes involving the conversion of a wide variety of polyalkyl-substituted aromatic compounds to the corresponding aromatic carboxylic acids have been extensively studied and documented in the prior art. Generally speaking, all of these methods produce directly the fully oxidized carboxylic acids.
Bruson et al in U.S. Pat. No. 2,806,059 disclose that diaryl methanes or diaryl ketones in which the aryl rings are substituted by alkyl groups and combinations of alkyl groups with carbonyl containing groups (i.e. aldehyde, acid, ester) can be oxidized in acetic acid solution with oxygen using, typically, cobalt salts as catalysts and aliphatic ketones as promoters, to the corresponding carboxylic acids.
Saffer et al in U.S. Pat. No. 2,833,816 disclose the preparation of aromatic polycarboxylic acids by oxidizing polyalkyl aromatic compounds with oxygen in solution (preferably acetic acid) using a heavy metal oxidation catalyst in combination with bromine either in elemental, combined or ionic form.
In U.S. Pat. No. 3,038,940, Serres et al disclose the oxidation of diaryl substituted methylene groups to the corresponding diaryl ketones in a solution of an oxidation resistant monocarboxylic acid in the presence of the combination of a heavy metal oxidation catalyst and bromine.
In U.S. Pat. No. 3,089,906 Saffer et al disclose a process carried out above atmospheric pressure wherein alkyl-substituted aromatic compounds are oxidized in solution in the presence of a heavy metal oxidation catalyst and a source of bromine.
Broadhead in U.S. Pat. No. 3,652,598 discloses the oxidation of various 2,2',3,3'- and 3,3',4,4'-tetraalkyldiphenylmethanes to the corresponding 2,2',3,3'- and 3,3',4,4'-tetracarboxylic acids by oxidation of the substrate with oxygen in acetic acid solution using, inter alia, manganese bromide as catalyst as taught in U.S. Pat. No. 2,833,816 cited supra.
Jones et al (U.S. Pat. No. 3,162,683) in reporting on the oxidation of alkyl aromatic compounds in the presence of perhalogenated aliphatic carboxylic acids noted, in the case of o-xylene, that o-toluic acid was formed along with varying proportions of phthalide. However, neither the complete oxidation of xylene to phthalide, nor the oxidation of both methyls of the xylene to carboxyl groups was found.
In the typical prior art cited supra, the methods for oxidizing the polyalkyl-substituted aromatic compounds provide satisfactory yields of the aromatic acids when the alkyl groups are not on adjacent carbon atoms of the same aromatic ring. However, where the alkyl groups are in ortho relationship to each other (i.e. on adjacent carbon atoms of the aromatic ring) the overall yields of the fully oxidized products produced by these prior art methods are low.
Apparently, the oxidation of the first alkyl group to a carboxylic group has the effect of deactivating the adjacent alkyl group thereby slowing down, or stopping completely, the oxidation of the second alkyl. Generally speaking, product mixtures are obtained which contain mono-, di-, tri-, or tetracids depending on the starting number of alkyl groups and the extent of oxidation. For example, when oxidizing the 2,2',3,3'- or 3,3',4,4'-tetraalkyldiphenylmethanes set forth in the process described in U.S. Pat. No. 3,652,598 cited supra the corresponding pure tetracids are not obtained but rather mixtures comprised of the mono-, di-, tri-, and tetracids. Consequently, yields of the desired tetracid are lowered and purification steps become complicated.
These disadvantages have been partially overcome in the prior art by resorting to much more rigorous oxidation conditions in terms of the reagents employed as typically disclosed in U.S. Pat. Nos. 3,078,279 and 4,173,573 which call for the use of nitric acid at elevated temperatures and pressures (i.e. 110.degree. C. to 350.degree. C. and up to 500 pounds per square inch). While yields of desired tetracids are superior to those from the other methods discussed above, the more stringent operating conditions required because of the nitric acid under the reaction conditions of high temperature and pressure make for an expensive and somewhat dangerous procedure.
Surprisingly, it has now been discovered that a certain class of phthalide compounds can be oxidized to the corresponding aromatic carboxylic acids under mild ionic oxidizing conditions of aqueous alkaline hypohalite. The yields and product purity in regard to fully oxidized products are superior to the direct autoxidations of the prior art discussed above yet the conditions are far less stringent than those prior art methods which employ nitric acid.
The only reference of which I am aware concerning a related method is U.S. Pat. No. 4,323,700 wherein a different class of phthalides are oxidized under ionic conditions to products unrelated to the instant orthodicarboxylic acids.
Further, it has also been discovered that, when a certain class of aromatic compounds having dialkyl groups substituted on adjacent carbon atoms of an aromatic ring are subjected to autoxidation techniques similar to those described above in the prior art but differing in one key respect, the result is not the direct formation of the corresponding polycarboxylic acid but rather of an alkanoyloxyphthalide. The key difference is the carrying out of the autoxidation in a solution of an aliphatic carboxylic acid anhydride. The anhydride plays more of a role than just a solvent and this role will be discussed in detail below.
Furthermore, the above steps can be combined to provide an improved method for oxidizing the aromatic compounds referred to above to the corresponding aromatic polycarboxylic acids in yields and product purity which exceed the prior art direct autoxidation methods while at the same time avoiding the use of nitric acid.
In a further unexpected advantage to flow from the combination of the two methods in accordance with the present invention, it has been found that the ionic oxidation proceeds under even milder conditions (circa 20.degree. C. and below) when the crude reaction mixture from the autoxidation is employed without purification other than removal of solvent when compared to oxidizing an isolated purified form of the phthalide. This aspect of the present invention will be discussed in detail below.